1. Field of the Invention
This invention generally relates to a flexible, radiation-resistant heat barrier which is capable of substantially blocking convection currents of a relatively hot gas from entering a cooler atmosphere, but yet is gas permeable. It also reduces heat transfer due to thermal radiation and conduction between the two regions. It is particularly useful in liquid metal nuclear reactor plants for insulating the relatively cool atmosphere within the reactor cavity from the hot gas convection currents generated in the annular space between the reactor vessel and the guard vessel.
2. Description of the Prior Art
Heat barriers for use in liquid metal reactor plants are known in the prior art. Generally speaking, such heat barriers cover the annulus between the reactor vessel and the guard vessel in order to prevent the free convection of hot gases generated in this annulus from entering the significantly cooler atmosphere within the reactor cavity. While it would be possible to operate a liquid metal reactor without such a thermal barrier, the convection currents generated by the almost 900.degree. F. outer wall of the reactor vessel would greatly increase the heat load on the reactor cavity cooling system. Since the difference in calculated heat load for a typical plant with and without such a device is 60,000 vs. 500,000 BTU's/hour, the provision of such a heat barrier is a highly desirable, if not essential, feature.
In the prior art, such heat barriers have generally been formed from an overlapping configuration of sliding, segmented insulating panels mounted on the top edge of the guard vessel. Each of these panels is spring loaded against the wall of the reactor vessel to accommodate differential thermal expansion between the reactor vessel and the guard vessel. The panels which engage the reactor vessel are made from rigid insulation so that their vessel engaging edges will not abrade the reactor vessel and will resist wear from the friction 10 they experience from the longitudinal and lateral differential thermal expansion between the guard vessel and the reactor vessel.
Unfortunately, such prior art thermal barriers are mechanically complex (somewhat like the iris diaphragm of a camera), and have the potential for springing a thermal leak due to wear or binding of the sliding panels or from the breaking of a spring. Since such thermal barriers are located in an area around the reactor vessel that is virtually inaccessible after plant startup due to the very high ambient radiation levels, repairs on such prior art barriers are extremely difficult, if not impossible, to perform. While it has been proposed to obviate the problems associated with such prior art barriers by eliminating the barrier and partially filling the annular space between the reactor vessel and the guard vessel with some sort of insulation materials, this annulus must be kept free so that it offers an unimpeded flowpath for any liquid sodium which may leak into the guard vessel. It must also be kept free to allow the welds of the vessels to be routinely inspected by a wheeled, remote camera that crawls in the annulus.
Clearly, there is a need for an improved thermal barrier which is capable of reliably insulating the reactor cavity from the hot convection currents generated by the outer walls of the reactor vessel for the lifetime of the plant with no planned maintenance. Ideally, such a barrier should be simple and relatively inexpensive in construction, and easy to install on virtually any of the existing liquid metal reactor designs with a minimum of extra weldments on the guard vessel or reactor vessel. Finally, the barrier should be able to block such convection currents uniformly throughout its area despite the substantial dimensional changes between the guard vessel, the reactor vessel, and the rest of the reactor facility by virtue of thermal differential expansion, and should also have some degree of gas permeability so that it will not break or deform in respnse to a differential gas pressure between the annulus and the reactor cavity which would occur during reactor heat-up, cool-down or in the event of a major sodium leak into the annulus.